1. Field of the Invention
This invention relates to an ignition timing control system for an internal combustion engine, for controlling ignition timing of the engine with a predetermined feedback control algorithm.
2. Description of the Related Art
Conventionally, as an ignition timing control system of this kind, the present assignee has already proposed an ignition timing control system disclosed in the publication of Japanese Patent No. 3779075. In the internal combustion engine, intake air amount control for increasing the amount of intake air is carried out during idling of the engine, and ignition timing is controlled by an ignition timing control system. In the ignition timing control, the rotational speed of the engine and a target rotational speed are set to a control variable and a target value thereof, respectively, and ignition timing as a control input is controlled with a predetermined feedback control algorithm such that the controlled variable converges to the target value. In this case, as the predetermined feedback control algorithm, there is employed an algorithm to which is applied a sliding mode control algorithm.
Further, in the control of ignition timing of the engine, a method is known which controls the ignition timing using pressure within a cylinder of the engine, i.e. in-cylinder pressure as a controlled variable.
According to the former ignition timing control system, the ignition timing is controlled with the predetermined feedback control algorithm such that the controlled variable converges to the target value. Therefore, when the in-cylinder pressure containing a lot of high-frequency noise components is used as a controlled variable as in the latter ignition timing control method, the ignition timing is feedback-controlled in an oscillating fashion, which can increase combustion variation. To sidestep this problem, it is envisaged to cause the controlled variable to be subjected to a low-pass filtering process or a moving average process. In this case, when there is hardly any change in the target value as in a steady operating condition of the engine, it is possible to control the controlled variable to the target value or its vicinity whereby control stability can be achieved, whereas when there is a large difference between the controlled variable and the target value as in a transient operating condition of the engine, there is a possibility that the follow-up property of the controlled variable to the target value is lowered.